Most aircraft today use aluminum, both in the interior structure and on the exterior. Frequently, it is necessary to attach articles, such as fittings, antennas, aluminum parts, etc., to the aluminum structure on the aircraft. These items are attached by known methods, including welding, bonding with an adhesive, etc. Further, aluminum sometimes becomes damaged and must be repaired. The damage may be a hole, a scrape, or other damage requiring repair. Repairing damaged aluminum structure often requires attaching or bonding on a repair plate.
The aluminum surface to which the article is to be attached must be properly cleaned and prepared prior to attachment to ensure a tight bond. One method presently used to treat aluminum prior to attachment is phosphoric acid anodizing. Phosphoric acid anodizing cleans dirt and oxide layers from the aluminum and etches the aluminum to provide a clean and bondable surface.
To perform phosphoric acid anodizing, the aluminum surface to be treated must be covered with phosphoric acid. It is critical that the entire surface be covered with acid and that no air pockets exist, thus preventing the acid from contacting the aluminum.
According to present methods used in repairing and constructing an aircraft, a large tank, many feet deep and many feet long, is filled with phosphoric acid. The part to be treated is placed in the phosphoric acid tank. This ensures that the acid fully contacts the surface of the aluminum.
After the aluminum part is in the tank, a negative voltage is placed on the part and a positive voltage is placed on a metal screen away from the surface to be treated. This creates a voltage potential difference between the two conductors, with acid therebetween. Current flows through the acid, completing the circuit, to perform phosphoric acid anodizing. After the prescribed time at the prescribed voltage for the part, as is well known in the art of phosphoric acid anodizing, the voltage is removed and the part is taken from the tank. The surface of the part is now prepared for attachment as by bonding, welding, etc.
The tank for holding the phosphoric acid must be large enough to hold the largest part which is expected to be placed therein. The tanks are often made large enough to hold entire sections of aircraft, including portions of wings, rudders, tail sections, etc. This requires many disadvantage of large-tank, phosphoric acid anodizing.
One advantage of the tank method is that all exposed surfaces, without respect to orientation when placed on the aircraft, can be anodized and prepared. The disadvantage is that the part must be small enough to fit into the tank, or alternatively, the tank must be very large.
When an aircraft is fully assembled and has been in use for several years, it is often necessary to perform a repair on the aircraft or bond an item to the aircraft. It is necessary to properly prepare the aluminum surface prior to bonding the item, such as by phospheric acid anodizing, as described herein. If the tank method of anodizing is used, it is necessary to remove that portion of the aircraft which is to be treated and send it to a repair facility having a large tank so that the part may be properly treated prior to attaching the item. This creates a disadvantage, both in turnaround time and repair cost. It also requires partial disassembling of the aircraft, sending the part to be repaired to a tank, and waiting for return shipment. This is a significant disadvantage for a military aircraft, which may be damaged in service at a site remote from an anodizing tank.
One method of performing phosphoric acid anodizing in the field, in-situ on the aircraft, is known as the Phos-Acid Non-Tank application (PANTA). PANTA involves placing a damming barrier around the surface to be treated. The barrier is filled with phosphoric acid, and an insulation is placed over the area to be anodized. A metal screen or other suitable conductor is then placed in the phosphoric acid. A voltage potential difference between the two conductors is created, causing current to flow between the conductors through the acid to perform the anodizing. After the anodizing is completed, the acid is removed from the barrier and the barrier is removed from the surface of the plane. The surface area just treated is now ready for priming and attaching an item, as in a bonded repair, welding, etc. While PANTA may be used in the field, it has numerous disadvantageous. A major disadvantage of PANTA is that it can be used only on an upper horizontal surface, such as on the top of a wing. This is a significant limitation and any surfaces that are vertical or on the underside of an aircraft must still be removed from the aircraft and sent to a repair facility for tank anodizing.